Aberration of starlight
Because the Earth moves through falling starlight, stars appear shifted slightly towards the direction of travel, like rain that seems to slant when you run.
The aberration of starlight is a small shift in where every star appears, caused by the Earth's motion around the Sun. Starlight arrives at a finite speed while we move sideways through it at about 30 km/s, so a telescope has to be tipped very slightly forward to catch it, just as you tilt an umbrella forward when you run through vertical rain.
The shift is tiny, at most about 20 arcseconds, roughly the width of a coin seen from 250 metres away, and it swings back and forth over the year as the Earth's direction changes.
James Bradley explained it in 1729. Since the size of the shift depends on the ratio of the Earth's speed to the speed of light, it gave light about 8 minutes and 12 seconds to travel from the Sun, confirming Rømer's idea from 1676.
A common misconception is that aberration is the same as parallax. Parallax depends on a star's distance; aberration is the same for all stars and depends only on our velocity.
